1. Field of the Invention
The present invention relates to an FSAN (Full Service Access Network) and an ATM-PON (Asynchronous Transfer Mode-Passive Optical Network) which is under international standardization by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector), and particularly to a dynamic bandwidth assignment system and a dynamic bandwidth assignment method for carrying out dynamic bandwidth assignment of the uplink band from a network termination to a network unit in these networks.
2. Description of Related Art
FIGS. 8A and 8B are block diagrams showing a configuration of a conventional dynamic bandwidth assignment system disclosed in Japanese patent application laid-open No. 7-135502, for example. In these figures, reference numerals 12a, 12b and 12c each designate user equipment; 14a, 14b and 14c each designate a transmission line; 15 designates a distributor; 16 designates a transmission line; and 17 designates a network unit. The user equipment 12a, 12b and 12c is connected to the distributor 15 via the transmission lines 14a, 14b, 14c, and the distributor 15 is connected to the network unit 17 via the transmission line 16. FIG. 8A shows the flow of downlink signals from the network unit 17 to the user equipment 12a, 12b and 12c, whereas FIG. 8B shows the flow of uplink signals from the user equipment 12a, 12b and 12c to the network unit 17.
Next, the operation of the conventional system will be described.
In FIG. 8A, the network unit 17 transmits a frame identifier F, followed by supplying the transmission line 16 with information a to the user equipment 12a, information b to the user equipment 12b, and information c to the user equipment 12c. The information a, b and c consists of information items the network unit 17 receives from a communication network. The network unit 17 provides the information items a, b and c with user equipment information about the destinations.
The distributor 15 supplies all the transmission lines 14a, 14b and 14c with the same signal. The user equipment 12a, 12b and 12c each receives the signal from the transmission lines 14a, 14b and 14c, and extracts the information addressed to itself. The frame identifier F in FIG. 8A has information indicating the time slots to be used in the next uplink frame by the user equipment 12a, 12b and 12c as shown in FIG. 8B.
In FIG. 8B, the user equipment 12a, 12b and 12c each recognizes its own time slots from the frame identifier F in FIG. 8A, and supplies the time slots with the information items a, b and c, respectively.
Incidentally, in order to increase the number of time slots to be transmitted, the user equipment 12a, 12b and 12c supplies the information to all the time slots assigned. As for the frame identifier F, it can be always transmitted by the user equipment 12a, for example.
The distributor 15 supplies the transmission line 16 with the information items a, b and c in this order. When the transmission lines 14a, 14b and 14c and the transmission line 16 consist of an optical fiber, an optical star coupler can be used as the distributor 15. The network unit 17 receives the frame identifier F and information items a, b and c from the transmission line 16, and transfers them to the communication network.
Receiving the entire information of a frame, the network unit 17 makes a decision as to whether all the time slots assigned to the equipment 12a, 12b or 12c of each user carry the information, and when the decision result is positive, it increases the number of the time slots assigned to the user equipment, whereas when the decision result is negative, it decreases or holds the number of the time slots, and the information about the increase or decrease is added to the uplink frame identifier F of FIG. 8A.
Thus, the network unit 17 can increase the number of the time slots to be assigned to the user equipment that requires the increase of the uplink time slots.
With the foregoing configuration, the conventional dynamic bandwidth assignment system has a problem of statistically increasing a cell transfer delay and a bursting tendency of a cell.
Here, the problem will be described in more detail taking an example of the user equipment 12a, which problem also holds true for the user equipment 12b or 12c. 
FIG. 9 is a diagram illustrating relationships between assignment positions of time slots and output information in reference to the necessary bandwidth of the user equipment 12a. In FIG. 9, frames 1 and 2 are named so for the convenience of explanation, and the frame identifier F is omitted from the output information of the user equipment 12a. 
FIG. 9 illustrates an example where the user equipment 12a makes a request for increasing its necessary bandwidth at a midpoint in the frame 1. As typical services that request an increase or decrease of the necessary bandwidth, there are such services as data transmission of videos, data transmission by computers, etc. In the example of FIG. 9, the user equipment 12a halts the output of the information at the assigned time slot before the request for increasing the necessary bandwidth because it does not need to increase the number of the time slots before the request, but after that it supplies the information to all the assigned time slots because of the increase of the necessary bandwidth.
However, it is not until the network unit 17 receives the final information in frame 2 from the user equipment 12a that it can recognize the request to increase the number of time slots from the user equipment 12a, because the user equipment 12a does not transmit its information in the time slot in frame 1 . Waiting for the arrival of the frame 2 in its entirety, the network unit 17 also recognizes the request to increase the number of time slots from other user equipment, and starts the reassignment of the time slots. When completing the reassignment, the network unit 17 provides the downlink frame identifier F with time slot information. Receiving the time slot information, the user equipment 12a can output its information to the reassigned time slots from the next uplink frame.
As described above, it takes one to two frame periods for the network unit 17 to start the reassignment of the time slots in response to the request to increase the necessary bandwidth from the user equipment. Generally, when the user equipment is not assigned necessary time slots, it stores the information to be transmitted in its buffer memory. Thus, the delay of the reassignment of the time slots will increase the capacity of the buffer memory, along with the amount of information to be stored in the buffer, resulting in an increase in the transfer delay. Besides, the increase in the number of cells stored in the buffer increases the bursting tendency of the cells. Since a larger capacity buffer memory is usually required to transmit the information with high bursting tendency through the communication network, it is preferable for the equipment of the communication network to transmit information with low bursting tendency.
Next, FIG. 10 is a diagram illustrating relationships between the number of time slots assigned to the user equipment 12a and the amount of the output information of the user equipment 12a when the necessary bandwidth of the user equipment 12a is increased and held thereafter. In FIG. 10, there are shown two delay times: a delay time X between the time when the user equipment 12a makes a request to increase the number of the time slots and the time when it actually outputs its information using the time slots with their number being increased, and a delay time Y between the time when the user equipment 12a halts the request to increase the number of the time slots and the time when it actually transmits its information using the time slots with their number being reduced. Thus, when the necessary bandwidth of the user equipment 12a is increased, the number of the time slots assigned to the user equipment 12a is increased after the time X, and hence the amount of the output information of the user equipment 12a is increased.
For convenience of explanation, it is further assumed that the bandwidth of the time slots whose number is increased satisfies the necessary bandwidth of the user equipment 12a, and that when all the time slots assigned are not supplied with the information, the number of the time slots assigned is decreased without exception.
Afterward, when the user equipment 12a transmits the entire information stored in the buffer memory, it does not supply all the assigned time slots with its information. In response to this, the network unit 17 decides that it can reduce the number of time slots to be assigned to the user equipment 12a, and actually reduces the number of the time slots after the time period Y. Thus, the amount of the output information of the user equipment 12a is further decreased, and the information is stored in the buffer memory, again. The operation is repeated until the necessary bandwidth of the user equipment 12a is reduced.
The delay time increases with an increase of the amount of information in the buffer memory. The foregoing operation brings about the states alternately where a large amount and a small amount of information is transmitted, thereby increasing the bursting tendency of the cells.